Known casting techniques are degraded due to air entrapment within the molten metal material introduced into a casting mold. The entrapped air forms pores in the molded article which weaken the structure and which degrade the metallurgical integrity of the article. In known casting methods, when a volume of molten metal material within a mold cools, it shrinks as it transforms from a molten state to a solid state. Pores result from air entrapped in the mold and the shrinking of the molten material increases the difficulty of obtaining near net shape castings. Shrinkage changes the size of the article and results in void cavities and pores in the article, thus making it unheatreatable. This results in lower mechanical properties and an inferior product. Near net shape castings are casting which require little or no finishing or machining for the articles to be of a predetermined shape and size.
It is known to use pressure to force air from a mold as well as to minimize the space occupied by the air to thereby decrease the size of the resulting pores in the cast article. One known die casting method includes a plunger which extends into a central region of the mold cavity to densify the metal proximate to the plunger. The pressure from the plunger compresses the molten metal to densify the material and reduce air spaces or voids within the material. The known die casting devices, such as that which utilize a central plunger, effect greater densification of the metallic material proximate the plunger than material which is spaced apart from the plunger. This results in uneven density in the cast article.
It is desirable to produce near net shape cast metallic articles which have a uniform density and which have desirable metallurgical properties, such as like those found in forgings. It is further desirable to be able to cast near net shaped articles having more complicated geometries than is normally allowed in forging operations wherein the metallurgical properties of the article are desirable and which rival that of a forged part.
The prior art utilizes complicated tooling which provides for relative movement within the die members during the densification process, i.e, the densification member moves relative to one of the die members. This requires replacement of the densification members when the tooling is replaced. One embodiment of the present invention overcomes this disadvantage by moving the platen and die member mounted thereon by the densifying assembly which allows replacement of the tooling without modification of the densifying assembly. This represents a substantial savings over prior art constructions which require modification of the densifying assembly every time a new die member is placed in the squeeze casting apparatus.